This invention relates to optical couplers, and, more particularly, to systems and methods for micro-optic couplers incorporating fiber tapers.
Cladding-pumped fiber lasers and optical amplifiers that use a fiber-coupled multimode laser diode to pump a gain medium consisting of a singlemode core embedded in a multimode waveguide have been increasingly common as a means of producing singlemode fiber lasers and fiber amplifiers with high output powers. After the early work of Maurer, (U.S. Pat. No. 3,808,449, issued in 1974), the concept was refined by Snitzer et al., (U.S. Pat. No. 4,815,079, issued in 1989), Gapontsev et al. (Patent No. WO96/20519 issued in 1996), DiGiovanni (U.S. Pat. No. 5,659,644, issued in 1996) and many others.
One characteristic feature of cladding-pumped fiber lasers is that the pump absorption decreases as the area of the multimode waveguide increases. It is therefore desirable to use the smallest waveguide area that is consistent with the brightness of the pump source. As a result, the double-clad fibers tend to be small diameter fibers with large numerical apertures. This allows the absorption to be maximized, and minimizes the required fiber length. It is also true that for the same brightness, the optimum coupling efficiency for the multimode pump sources usually is obtained with a larger fiber and a smaller numerical aperture. This is a result of matching the fiber diameter to the emitting aperture of the multimode diodes. This situation results in a need for some form of beam reshaping in the optical system connecting the pump lasers with the cladding-pumped device. A variety of imaging and non-imaging optical devices can be used to make such transformations, including both GRIN lenses and fiber tapers, as has been recognized in some of the optical systems proposed in the prior art. For example, DiGiovanni et al. U.S. Pat. No. 5,659,644 (""644 Patent) shows a combined taper and coupler in which a seven-fiber hexagonal bundle is drawn down by a factor of two to convert six low-divergence pumps into a single high-divergence source that matched their double-clad waveguide geometry.
One form of pump-signal beam coupler is based on the micro-optic coupler commonly used as singlemode devices. Typical examples of such devices include the Fiber-Wave Length Division Multiplexer (FWDM) and Isolator-Wave Length Division Multiplexer (IWDM)-series devices made by E-Tek Dynamics of San Jose, Calif. A similar device using a multimode fiber is described by the DiGiovanni et al. ""644 Patent. These devices use GRIN lenses and a dichroic reflector to collimate the inputs from a pump fiber and a signal fiber and then to image them into a common output fiber. As imaging systems, these devices work equally well with either singlemode or multimode fibers. They can be turned into components for cladding pumped devices by replacing the pump and common fibers with double-clad fibers that permit simultaneous propagation of a singlemode signal and a multimode pump. Such devices are described in an earlier patent application entitled xe2x80x9cMultifunctional Optical Isolatorsxe2x80x9d that is incorporated herein by reference. If singlemode fibers are spliced to the ends of the double-clad fibers, they serve as mode filters and allow the device to be aligned as if it were a singlemode device. Not only is this compatible with the manufacturing of the existing parts, but it also ensures that the fibers can be readily aligned with sub-micron precision. The singlemode core in the pump fiber does not affect the propagation of the multimode pump beam, which propagates primarily in the outer waveguide of the fiber. The double-clad fiber can be regarded as a complex multimode fiber, with an outer core diameter defined by the outer waveguide with the added core having no deleterious effect on the performance of the device. Using a simple multimode fiber (i.e. a fiber with the same outer core diameter but without the singlemode core) is also possible. This was as suggested by DiGiovanni et al. in the ""644 Patent, but has the disadvantages that it increases the number of fiber types in the device and that singlemode alignment procedures can no longer be used.
The efficiency of such devices will be maximized if multiple functions can be applied to the beam passing through the device. The signal beam can be most easily addressed in designs where the pump fiber and the output fiber are attached to one end of the device and where the signal wavelength is transmitted through the device. In particular, an isolator or a filter can be incorporated into the signal beam path, as was discussed in an earlier application, and as is done in the previously-mentioned E-Tek IWDM device. Transmitting the pump beam through the device allows pump beam reshaping to be accomplished, as noted by DiGiovanni et al. in the ""644 Patent. Unfortunately, existing devices do not allow both beams to be manipulated in the device, so that a device with an isolator and pump beam reshaping cannot be built.
Since it is highly desirable to incorporate a device such as an isolator into the signal beam path, it is desirable to identify alternative means for reshaping the pump beam. Existing micro-optic couplers are simple and robust, but in the reflected channel they are unity-magnification systems that do not provide the desired beam shaping. Beam shaping can be accomplished with an external component, such as a GRIN lens or fiber taper, placed between the pump diode and the coupler. Such an approach is undesirable because it adds a component, introduces losses and in the case of the fiber taper, entails a rather difficult splice between a precisely fabricated taper and smaller-core fiber in the coupler.
Directly incorporating the beam shaping into the pump-signal coupler is desirable because it simplifies the device. It is also desirable because it reduces the number of splices associated with the device. In general, minimizing the number of splices will also enhance the efficiency. This applies to the splice to an external beam-shaping optic as well as the splice between the output fiber and the gain fiber. Devices that minimize the number of splices by directly incorporating both a beam reshaping optic (such as a taper) or a gain fiber are likely to maximize efficiency.
Accordingly, a need exists for systems and methods for efficient and multi-functional micro-optic couplers that can incorporate isolators or filters into the signal path, pump beam reshaping optics into the pump path and double-clad gain fiber into the common path.
This invention addresses the prior problems and provides for systems and methods of reducing loss in devices utilizing double-clad gain fibers.
This invention incorporates one or more tapered multimode optical fibers into a micro-optic coupler to manipulate the shape of a multimode beam. In one embodiment, the invention is used with a singlemode signal beam, and is configured to include a collimated free-space region where a device such as an isolator can be placed in the signal beam without interfering with the pump beam. In this embodiment, the invention further provides for a double-clad fiber whose multimode section is matched to the output of the optical taper and whose singlemode core is coupled to the singlemode signal beam. This double-clad fiber can be an undoped fiber spliced to a double-clad gain fiber, or it can be the gain fiber itself. In an alternative embodiment, the outputs of two, tapered multimode fibers can be combined into a single, reshaped multimode output.
For purposes of redundancy, it is often desirable to combine two pump sources in an amplifier. Two sources at different wavelengths can be combined using a dichroic reflector in a unity-magnification micro-optic coupler with identical multimode fibers at the three fiber ports. The output of this pump combiner can then be coupled to the pump-signal combiner, where a taper could be used to reshape the combined beams for maximum efficiency. Tapers could also be used at the two inputs to the pump combiner, so that beam shaping would occur in the pump combiner instead of the pump-signal combiner. This represents an alternative embodiment of the invention that can be useful for retrofitting devices where a pump-signal combiner with non-tapered fibers is already in use.
One advantage of this invention is that it provides a way of incorporating beam-shaping optics into a micro-optic coupler without changing the other features of the device. For a double-clad beam combiner, it allows the incorporation of beam-shaping elements without perturbing the collimated signal beam path where isolators or filters are incorporated. For a multimode beam combiner, it allows beam shaping to be applied to both multimode inputs without changing the design of the unity magnification device. Another advantage of the invention is that the use of internal beam shaping optics eliminates the substantial inefficiencies that occur if the beam shaping elements are not used, without the loss associates with external beam shaping optics. Another advantage of this invention is that it allows devices that incorporate the gain fiber directly, eliminating a potentially troublesome double-clad splice. Yet another advantage of this invention is that incorporating the taper into the coupler reduces the mechanical constraints on the taper, and makes the taper fabrication process less demanding.
Other objects and advantages of this invention will become apparent to those skilled in the art upon review of this document, the Figures and the claims.